1. Field of the Invention
The invention relates to a circuit board, and more particularly, to a circuit board having a photo-imaginable dielectric layer.
2. Description of Related Art
A common non-embedded circuit board has been developed into an embedded circuit board in the field of circuit board technologies today. Specifically, the common non-embedded circuit board is characterized in that circuits therein protrude onto a surface of a dielectric layer, whereas the embedded circuit board is characterized in that circuits therein are embedded in the dielectric layer.
In the embedded circuit board of single-layer, an insulation material between traces of the circuit board mainly adopts use of a solder mask, for example. Therefore, other than serving as a solder resist material on a surface of the circuit board, the solder mask also serves as a dielectric insulation material between circuits of the circuit board. However, electrical insulation of the solder mask and thermal expansion coefficient (Coefficient of Thermal Expansion; CTE) of the material thereof are not as good as those of the traditional dielectric material. With development trend of electronic device towards miniaturization nowadays, as an area of the circuit board must be reduced correspondingly, a density of the traces on the circuit board must also be relatively increased. Still, as described above, because the electrical insulation of the solder mask in the embedded circuit board of single-layer is not as good as that of the traditional dielectric material, a minimum space between the traces is limited. Accordingly, a design rule for the traces of the circuit board also shows limitations which are disadvantageous in terms of further reduction on the area or thickness of the circuit board.